WATCH: Monkeying around takes unusual turn

Yakushima Island - After spending a few months observing macaque monkeys living in a Japanese forest, a group of scientists found a pattern of behaviour they described as both "unusual" and "intriguing."

Adolescent female macaques climbed on top of a sika deer and crouched. Then, they moved their pelvises as if thrusting or grinding. They squeaked sexual sounds. They also bit, sniffed and pulled on the deer's antlers.

The deer, meanwhile, stood nonchalantly and continued foraging for food.

When the deer walked away, the female monkeys "often displayed sexually motivated tantrums which consist of crouching on the ground, body spasms and screaming, while gazing at the deer," the scientists wrote in a peer-reviewed study published this week in the Archives of Sexual Behavior.

Scientists found that the interactions, documented in the Meiji Memorial Forest of Minoo Quasi-National Park on the outskirts of the city of Minoo in central Japan, are "sexual in nature" - at least for the monkeys. Japanese macaques are known to ride deer for play or to move from one place to another, but adolescent females appear to have taken this playful interaction a step further. Why they do so remains unclear, but scientists have a few theories.

"The monkey-deer sexual interactions reported in our paper may reflect the early stage development of a new behavioral tradition at Minoo," Noëlle Gunst, co-author of the study and a psychology professor at the University of Lethbridge in Canada, told the Guardian.

Whether this is a "short-lived fad" or the beginning of a "cultural phenomenon" also is hard to tell without further research, the authors wrote.

Scientists observed a total of 170 male and female Japanese macaques. Eighteen were adolescent females ages 3 to 4. Over the course of several weeks, scientists observed only adolescent females engaging in sexual activities - with one another and with sika deer.

Specifically, adolescent female macaques initiated sexual interaction with a sika deer 258 times. Of these, 13 sexual interactions initiated by five macaques were considered successful. That is, they had a "temporary, but exclusive, sexual association" with the deer. Almost all of the successful interactions involved an adult male deer or a stag, which seemed to passively oblige. Adolescent male and adult female deer, meanwhile, largely avoided the persistent primates.

In five instances, adolescent female macaques intruded on a fellow female's sexual interaction with a deer and successfully displaced the original "mounter," the authors wrote.

Scientists believe that the stags' tolerance was mainly driven by nothing more than potential hygienic benefits. While sitting on top of the stags, the monkeys groomed their backs and head.

The authors have a few possible explanations as to why adolescent female macaques engage in such behavior.

Like humans, adolescent female macaques go through a period in which they begin to develop sexual behaviours. It's likely that these interactions with deer mates was their way of practicing and developing their sexual behaviors, the authors wrote. These interactions may also be a substitute for their current lack of sex with those of the same species. Adolescent female macaques are not the preferred partners of adult male macaques and are often rejected.

"Consequently, adolescent females may seek stag mates as an outlet for sexual frustration," the authors wrote.

It's also possible that they prefer to have sexual interactions with fellow female macaques and passive stags over more aggressive males of the same species.

Or it may not be entirely about sex - at least not at first.

Adolescent macaques usually climb on top of sika deer to play or be transported. The females may "experience genital stimulation" during these "playful interactions."

"Then, during the surge of sex steroid hormones characteristic of the adolescence period, they may seek similar sexual reward with deer mates," particularly when they're sex-deprived and rejected by male macaques, the authors wrote.

The study followed a shorter one that was published earlier this year. The report, published in January, was about a mating behavior between a male Japanese macaque and a female sika deer documented on Yakushima Island in Japan.

The authors of the earlier report also hypothesized that the male Japanese macaque was likely sex-deprived and had limited access to females of the same species.